Process for the formation of an amorphous silicon deposited film with intermittent irradiation of inert gas plasma

ABSTRACT

A process for forming a silicon-containing amorphous film on a substrate which comprises (a) step of depositing a silicon-containing amorphous film on said substrate and (b) step of irradiating plasma of inert gas to said silicon-containing amorphous film on deposited on the substrate in said step (a), wherein said step (a) and said step (b) are alternately repeated.

BACKGROUND OF THE INVENTION

1. Field of The Invention

The present invention relates to an improved process for forming a largearea amorphous silicon deposited film excelling in semiconductorcharacteristics which includes the step of intermittently irradiatinginert gas plasma during film formation.

2. Related Background Art

As for the so-called amorphous silicon (a-Si in other words), there arevarious advantages, which cannot be attained in the case of a singlecrystal silicon, such that a film composed of a-Si (hereinafter referredto as a-Si film) can be formed not only on a glass substrate but also onother commercially available substrates at low substrate temperature;the a-Si film can be easily formed at large area; the a-Si film issuperior to a film composed of the single crystal silicon (hereinafterreferred to as single crystal Si film) with respect to light absorption;and the property of the a-Si film is isotropic but does not exhibitpolarity. In addition, the a-Si film is free of grain boundary which ispresent in a polycystal silicon film has. Further as for the a-Si film,there is an advantage that it can be relatively easily produced at areduced production cost.

In view of these advantages, there are a number of proposals to use ana-Si film as a constituent element in solar cells, in scanning circuitsof image-reading devices such as line photosensors, area photosensors,etc., in TFTS, TFT arrays or matrices used not only in operationcircuits of liquid crystal displays but also in switching circuits ofphotosensors, and electrophographic photosensitive members.

As the method of forming an amorphous silicon deposited film usable insuch semiconductor devices, there are known or reported plasma CVDmethods such as RF plasma CVD method (so-called glow dischargedecomposition method), microwave plasma CVD method, etc., reactivesputtering method, light-induced CVD method, thermal-induced CVD method,vacuum evaporation method and electron cyclotron resonance CVD method.

In order to form an a-Si film to be used as a constituent semiconductorlayer of an amorphous silicon semiconductor device by means of theplasma CVD technique using silane gas such as SiH₄, Si₂ H₆, etc. as thefilm-forming raw material gas, it is generally recognized that the RFplasma CVD method and the microwave plasma CVD method are appropriate.The reactive sputtering method in which a Si-target is sputtered withinAr plasma in the presence of hydrogen gas is also recognized as beingappropriate in order to form such a-Si film.

Other methods, that is, the light-induced CVD method, thermal-inducedCVD method, vacuum evaporation method and electron cyclotron resonanceCVD method, are practiced only in experimental scale but not employed inindustrial scale.

Now, the a-Si films to be used in semiconductor devices which are formedaccording to these film-forming methods are mostly hydrogenated a-Sifilms containing 10 atomic % or above of hydrogen atoms. In other words,it is considered that a-Si films usable as the electronic constituentmaterials exhibiting the characteristics required for obtaining a-Sisemiconductor devices are mostly those containing 10 atomic % or aboveof hydrogen atoms.

In order to form such a-Si films, the foregoing plasma CVD technique iswidely used in various sectors since there are various advantages thatthe constitution of the apparatus used is relatively simple and caneasily designed and the film-forming conditions can be relatively easilycontrolled. In the case of forming such a-Si film by means of the plasmaCVD technique, the film formation is carried out, for example, in thefollowing manner. That is, SiH₄ gas or Si₂ H₆ gas (if necessary, dilutedwith hydrogen gas) as the film-forming raw material gas is introducedinto a deposition chamber containing a substrate on which a film is tobe formed, wherein energy of a high frequency of 13.56 MHz or 2.45 GHzis applied to cause plasma by which the film-forming raw material gas isdecomposed to produce active species, thereby causing the formation ofan a-Si deposited film on the substrate. In this case, the resultinga-Si film can be easily made to be of n-type or p-type by mixing anproper doping gas such as PH₃, B₂ H₆, BF₃, etc. with the film-formingraw material gas.

In any case of forming an a-Si semiconductor film, it is known thatespecially the substrate temperature, among other film-formingparameters, greatly influences the quality of the resulting a-Sisemiconductor film and it is extremely important to maintain the entireof a substrate uniformly at a predetermined temperature particularly inthe case of forming a desirable a-Si semiconductor film of large area.However, it is very difficult to maintain all of a large area substrateuniformly at a predetermined temperature in order to form such largearea a-Si semiconductor film especially by means of a plasma CVD method.That is, in the case of forming a desirable large area a-Sisemiconductor film on a large area substrate, for example, of 100 mm to1000 mm in size by means of the plasma CVD method, wherein the filmformation is usually performed at a inner pressure of 0.1 to 1 Torr andat a predetermined substrate temperature in the range of 200° to 300°C., it is difficult to maintain the entire substrate of such large areasize uniformly at said temperature under the condition of such low innerpressure and it takes a long period of time until the predetermineduniform substrate temperature is attained for the entire of the largearea substrate. And in the case of repeatedly forming a large area a-Sisemiconductor film while replacing the previous large area substrate bya new large area substrate in each film forming operation, it isdifficult to maintain all of the new large area substrate uniformly atthe identical substrate temperature in each case. In order to repeatedlyform a desirable large area a-Si semiconductor film, other film-formingparameters than the above substrate temperature are necessary to beproperly controlled so that uniform plasma is produced along the entiresurface of a large area substrate on which the large area a-Sisemiconductor film is to be formed in each film-forming operation.However, it is difficult to control those parameters uniformly in therespective film-forming operations. Thus, it is extremely difficult tomass-produce a desirable large area a-Si semiconductor film having auniform film property at a high yield. Particularly, each of theresulting large area a-Si semiconductor films is liable to be such thatis varied with respect to the film property all over the large areasubstrate, and the resulting large area a-Si semiconductor films becomesuch that are varied with respect to the film property. This situationis problematic, for example, in the case of forming a multi-layeredsemiconductor device comprising a plurality of a-Si semiconductor filmsbeing stacked on a large area substrate. That is, it is difficult tomass-produce a desirable multi-layered semiconductor device each ofwhich constituent a-Si semiconductor films having a uniform filmproperty all over the large area substrate, which exhibits uniformcharacteristics and which is free of local occurrence of lightdeterioration (which is the so-called Stabler-Wronski effect) at a highyield, since to maintain all of the large area substrate uniformly at apredetermined temperature, to secure uniform distribution of plasmaalong the entire surface of the large area substrate and to secureuniformity of the film-forming conditions upon forming each of theconstituent a-Si semiconductor films are difficult as above described.

Particularly, the local occurrence of light deterioration at theconstituent a-Si semiconductor film is problematic even in the casewhere the above multi-layered semiconductor device is an image-readingdevice in which light having a relatively weak intensity is irradiated,but it is serious in the case where the above multi-layeredsemiconductor device is a solar cell in which sun light (which has astrong intensity) is irradiated. In the case where the abovemulti-layered semiconductor device is an electrophotographicphotosensitive member, the problem relative to unevenness in thecharacteristics of the constituent a-Si semiconductor film leads toreproducing undesirable images which are not even in density or whichare easily deteriorated. In the case where the above multi-layeredsemiconductor device is a thin film transistor (TFT), the problemrelative to unevenness in the characteristics of the constituent a-Sisemiconductor film makes the TFT such that does not exhibit uniform TFTcharacteristics. Similarly, in the case where a plurality of the abovemulti-layered semiconductor devices are arranged in matrix-like state tobe a device used in the operation circuit of a liquid crystal display,the problem relative to unevenness in the characteristics of theconstituent a-Si semiconductor film results in making said device suchthat it exhibits varied characteristics.

In order to solve the above problems caused by the a-Si semiconductorfilm formed by the conventional plasma CVD method, there is a proposalto improve its film property by subjecting the resultant a-Sisemiconductor to after-treatment. For instance, the preliminary reportfor the 1988 Autumn Conference of the Applied Physics Association5p-2f-1 and the preliminary report for the 1990 Spring Conference of theApplied Physics Association 31a-2D-8 & 31a-2D-11 report methods in whichthe previously formed a-Si film is subjected to repetition of H₂ plasmatreatment. According to these methods, the a-Si film is crystallized asthe H₂ plasma treatment is repeated, wherein the resulting Si film isnot amorphous but crystalline. The Si crystalline film is advantageousdepending upon application use, but the extent of its applicability isnarrower than that of an a-Si film. As for the crystalline Si film,there are disadvantages such that it is inferior to the a-Si film withrespect to light absorption; it is poor in uniformity because of havinggrain boundaries (the a-Si film is free of such problem); and it is notdesirable to be used in light receiving devices. In addition to thesedisadvantages, there is also a disadvantage that uncontrollablecrystallization is caused in the case of repeatedly treating the a-Sifilm with H₂ plasma.

Thus, the proposed methods are not effective in order to solve theforegoing problems in the prior art.

SUMMARY OF THE INVENTION

The principal object of the present invention is to overcome theforegoing problems in the prior art and to provide an improvedfilm-forming process which enables one to effectively mass-produce adesirable a-Si semiconductor film having an improved uniformity in filmproperty all over the entire film formed on a substrate.

Another object of the present invention is to provide an improvedfilm-forming process which enables one to stably and repeatedly producea large area a-Si semiconductor film having an improved uniformity infilm property all over the entire film formed on a large area substrate.

A further object of the present invention is to provide an improvedfilm-forming process which enables one to mass-produce an improved a-Sisemiconductor device which is free of occurrence of light deteriorationand which excels in photoresponse.

A further object of the present invention is to provide an improvedfilm-forming process which enables one to mass-produce an improved a-Sisemiconductor device which is free of occurrence of light deteriorationand which exhibits an improved photoelectromotive force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an example of thefilm-forming process according to the present invention.

FIG. 2 shows a typical transition curve illustrating the interrelationbetween a prescribed irradiation period (t_(A)) of inert gas plasma,change in the substrate temperature (Ts) and the hydrogen content of ana-Si film formed according to the film-forming process of the presentinvention, in which is also shown a comparative transition curve showingthe interrelation between change in the substrate temperature and thehydrogen content of an a-Si film formed according to the conventionalglow discharge decomposition method (GD method).

FIG. 3 shows two typical curves respectively illustrating theinterrelation between the thickness of an a-Si film deposited during aprescribed deposition period and the ratio between a peak belonging tocrystal material and a peak belonging to amorphous material in the Ramanspectrum of said a-Si film.

FIG. 4 shows two experimentally obtained curves respectivelyillustrating the interrelation between the period of time during whichirradiation of inert gas plasma is performed and the hydrogen content ofan a-Si film formed.

FIG. 5 shows two experimentally obtained curves respectivelyillustrating the thickness of an a-Si film formed while applying a biasvoltage upon irradiating inert gas plasma and the hydrogen content ofthe a-Si film.

FIG. 6 is a schematic diagram of an example of the film-formingapparatus which is suitable for practicing the process of the presentinvention.

FIG. 7 is a schematic cross-section view illustrating the constitutionof an element of a Schottky barrier type image-reading sensor formedaccording to the process of the present invention.

FIG. 8 is a schematic cross-section view illustrating the constitutionof a pin junction photovoltaic element formed according to the processof the present invention.

FIG. 9 is a schematic diagram of another example of the film-formingapparatus which is suitable for practicing the process of the presentinvention.

FIG. 10 is a schematic cross-section view illustrating the constitutionof a field effect thin film transistor formed according to the processof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be detailed by reference to the typicalembodiments which are provided for illustrative purposes. The presentinvention is not restricted by these embodiments. These embodiments canbe properly modified as long as the object of the present invention isattained.

The principal feature of the film-forming process according to thepresent invention which attains the foregoing objects comprisesalternately carrying out the step of depositing an a-Si film on asubstrate and the step of irradiating inert gas plasma to the previouslyformed a-Si film.

Particularly, the film-forming process according to the presentinvention is to form an a-Si semiconductor film on a substrate byalternately repeating the step of depositing an a-Si film on thesubstrate and the step of irradiating inert gas plasma to the a-Si filmdeposited on the substrate, wherein the a-Si film to be formed in therespective step of forming an a-Si film is controlled to be at least 10Å and the temperature of the substrate at the time of carrying out thestep of irradiating inert gas plasma to the a-Si film deposited ismaintained at a temperature which is corresponding to or higher than thetransition temperature (Tt) at which a change is caused in the hydrogencontent of the a-Si film.

The inert gas plasma includes plasma produced from an inert gas such asHe, Ne, Ar, Kr, Xe, etc.

In the present invention, a pronounced effect is provided in the casewhere a bias voltage is applied to the substrate side at least in therepetition step of irradiating inert gas plasma to the a-Si filmdeposited.

According to the film-forming process of the present invention, theforegoing problems in the prior art which are relative to localizationof the film property of a large area a-Si semiconductor film formed on alarge area substrate chiefly due to unevenness in distribution of thesubstrate temperature all over the substrate upon film formation areeffectively eliminated and as a result, a desirable large area a-Sisemiconductor film having a uniform film property all over the film andwhich is free of localization of the film property may be stably andrepeatedly formed by means of the plasma CVD technique even in the casewhere uniform distribution of a predetermined temperature is notattained all over the entire of the substrate on which the a-Sisemiconductor film is to be formed upon film formation and otherfilm-forming conditions are not severely controlled. Hence, according tothe film-forming method of the present invention, a large areamulti-layered a-Si semiconductor device each of which constituent a-Sisemiconductor films having a uniform film property all over the largearea substrate, which exhibits uniform semiconductor characteristics andwhich is free of local occurrence of light irradiation can bemass-produced at an improved yield.

The present invention is particularly effective in the production of alarge area a-Si semiconductor device such as a-Si solar cell, a-Si imagereading device, a-Si electrophotographic photosensitive device or TFTarray for use in the operation circuit of a liquid crystal display. Theprocess of producing such large area a-Si semiconductor device includesa variety of steps, for example, cleaning of a large area substrate,formation of an electrode, formation of an a-Si semiconductor layer ofp-type, i-type or n-type, lamination of these a-Si semiconductor layers,formation of a transparent electrode, pattering of the respective layerby means of photolithography for example, integration of devices,packaging, and the like. Among these steps in order to produce theforegoing large area a-Si semiconductor devices, extreme due care isnecessary to be made for the step of forming an i-type a-Si film to bethe i-type semiconductor layer since the resulting i-type semiconductorfilm often becomes one having varied characteristics due to occurrenceof unexpected factors other than the foregoing problems relative tounevenness in the substrate temperature during the formation thereof.However, the present invention eliminates such problems and makes itpossible to stably and repeatedly form a desirable i-type semiconductorfilm which exhibits uniform characteristics required for the i-typesemiconductor film.

Explanation in more detail will be made of the film-forming process ofthe present invention.

The film-forming process of the present invention comprises repeatedlyperforming a first step (a) of depositing an a-Si film for a prescribedperiod of time (t_(D)) and a second step (b) of irradiating inert gasplasma to the a-Si film deposited in the step (a) for a prescribedperiod of time (t_(A)) alternately. This situation is schematicallydiagramed in FIG. 1 in which are shown said prescribed period of timeduring which the a-Si film formed in the respective repetition step (a)is shown by t_(D) and t_(D) ', and said prescribed period of time duringwhich the irradiation of inert gas plasma is carried out is shown byt_(A) and t_(A) '. The repetition proceeding of the two steps (a) and(b) can be expressed by the following equations (1) and (2).

    L=v.sub.D ×t.sub.D ×n                          (1)

    t.sub.r =(t.sub.D ×t.sub.A)×n                  (2)

wherein, v_(D) denotes a deposition rate during the prescribed period oftime t_(D), n denotes the number of repeated cycles of the steps (a) and(b), and L denotes a thickness of the resulting a-Si deposited filmafter repetition of the steps (a) and (b) "n" cycles.

The mean deposition rate V_(D) can be expressed by the followingequation (3).

    v.sub.D =L/t.sub.T =t.sub.D /(t.sub.D +t.sub.A)×v.sub.D(3)

wherein, t_(T) denotes a period of time required for obtaining an a-Sideposited film having the thickness L.

In the film formation in practice, the respective values of L and V_(D)are substantially the same as or slightly smaller than those calculatedby the above equations.

The t_(D), v_(D) and t_(A) in each repetition cycle comprising the steps(a) and (b) are not always limited to such simple example expressed bythe above equations. That is, the t_(D), v_(D) and t_(A) may be properlychanged for each repetition cycle. Further, it is not always necessaryfor the v_(D) to be constant. The v_(D) may be a function of time.

Now, the surface of the a-Si deposited film formed on a substrate in thestep (a) receives irradiation of inert gas plasma in the step (b). Themechanism of how the surface of the a-Si deposited formed on thesubstrate in the step (a) is treated by the inert gas plasma irradiatedin the step (b) is not clear completely. But it is considered that ionsand/or excited atoms of the-inert gas in the inert gas plasma irradiatedcollide with the surface of the a-Si deposited film to activate thelattice vibration of the surface of the a-Si deposited film, wherebycausing release of excessive hydrogen atoms (H) and/or recombination orstructural relaxation of the Si network.

The thickness I (=v_(D) ×t_(D)) of the a-Si film deposited during thet_(D) is desired to be of about two atomic layers or more, andparticularly, it is desired to be 10 Å or more. In the case where ana-Si film newly deposited in the step (a) is excessively thin,particularly for example, of a thickness of one atomic layer, theamorphous structure is hardly maintained, wherein the excessively thina-Si deposited film is limitlessly crystallized with the irradiation ofinert gas plasma in the step (b). It is almost impossible to control thecrystallization degree in that case. In order to prevent the network ofthe a-Si deposited film from being excessively recombined and toeffectively attain desirable structural relaxation for the a-Sideposited film in the step (b), the thickness of an a-Si film depositedin the step (a) prior to the step (b) is necessary to be at least 10 Å.

The a-Si film deposited during the t_(D) in the respective repetitionstep (a) may partially contain microcrystalline Si as long as the a-Sifilm has a thickness of 10 Å or more. In this case, the a-Si depositedfilm partially containing microcrystalline Si will be structurallyrelaxed with the irradiation of inert gas plasma without beingundesirably crystallized because said a-Si deposited film has asufficient thickness of 10 Å or more.

In the case where the a-Si film deposited in the respective repetitionstep (a) is of a thickness exceeding 100 Å, the a-Si deposited film willnot be structurally relaxed as expected with the irradiation of inertgas plasma in the successive repetition step (b), even though theirradiation of inert gas plasma is excessively performed.

The structural relaxation degree of the a-Si deposited film which isconducted in the respective repetition step (b) may be confirmedaccording to reduction in the content of hydrogen atoms of the film orreduction in the half-width of the peak at 480 cm⁻¹ in Raman spectrum ofthe film.

In view of the above, the thickness of the a-Si film deposited duringthe t_(D) in the respective repetition step (a) is necessary to be lessthan 100 Å or most preferably, less than 50 Å.

In the case where a negative bias voltage is applied to the substrateupon irradiating inert gas plasma to the a-Si deposited film in therespective repetition step (b), it is possible for the a-Si filmdeposited in the respective repetition step (a) to be of a thicknesswhich is 1.5 to 2 times thicker over the above defined thickness. Thenegative bias voltage applied here may be a self bias voltage of theinert gas plasma. In this case, such self bias voltage may be applied byfloating the substrate from the earth.

One of the important factors in the film-forming process of the presentinvention is to maintain the substrate on which an a-Si semiconductorfilm is to be formed at a temperature which is corresponding to orhigher than the transition temperature (Tt) at which a change is causedin the content of hydrogen atoms of the film.

FIG. 2 shows a transition curve illustrating the interrelation betweenthe irradiation period of time (t_(A)) of inert gas plasma, change inthe substrate temperature (Ts) and the hydrogen content of an a-Si filmformed according to the film-forming process of the present invention.In FIG. 2, there is also shown a comparative transition curveillustrating the interrelation between change in the substratetemperature and the hydrogen content of an a-Si film formed according tothe conventional glow discharge decomposition method (GD method).

As FIG. 2 illustrates, it is understood that when the t_(A) (the periodof time during which the irradiation of inert gas plasma in therespective repetition step (b)) is sufficiently prolonged, there appearsa turning point which is not found in the film formation by theconventional GD method. It is considered that certain structuraltransition or certain change in the reaction mechanism has occurred atthis turning point. But no distinct reason has been clarified yet forthis. The important thing which can be understood from what shown inFIG. 2 is that the hydrogen content of an a-Si film formed in thesubstrate temperature region exceeding the transition temperature (Tt)is markedly lowered and the said hydrogen content does not substantiallydepend upon the substrate temperature in this region.

The present inventor experimentally found that an a-Si film formed inthe substrate temperature region exceeding the transition temperature(Tt) is relatively low with respect to hydrogen content. And the a-Sifilm has a spin density of less than 10¹⁶ cm⁻³ and has few danglingbonds (which means that the a-Si film has few defects in other words).

The present inventor obtained further findings through experiments. Thatis, (i) when the t_(A) (the period of time during which inert gas plasmais irradiated to the a-Si film formed in the respective repetition step(a) in the respective repetition step (b)) is sufficiently prolonged,for example, to about 60 sec., there is afforded an a-Si film whichcontains hydrogen atoms in an amount of 5 atomic % or below with noincrease in spin density; (ii) the hydrogen content of an a-Si film tobe formed can be properly controlled to a desired value by properlycontrolling the t_(A) ; When the t_(A) is sufficiently prolonged, thequality of an a-Si film formed is not changed to such an extent as thatfound for an a-Si film formed by the conventional GD method in the casewhere the substrate temperature is changed, because the dependency ofthe hydrogen content of the film upon the substrate temperature issubstantially slight in the substrate temperature region exceeding thetransition temperature (Tt) as apparent from what shown in FIG. 2;According the resulting a-Si film has a uniform film property all overthe film is free of occurrence of light deterioration (the so-calledStable-Wronski effect) and exhibits a desirable electric conductivitywhich is greater than that of an a-Si film formed by the conventional GDmethod. Therefore, (iii) even in the case of forming a large area a-Sisemiconductor film on a large area substrate, which is liable to causeuneven distribution of the substrate temperature upon film formation, inorder to produce a desirable large area multi-layered a-Si semiconductordevice, each of which constituent a-Si semiconductor films having auniform film property all over the large area substrate, which exhibitsuniform semiconductor characteristics and which is free of localoccurrence of light deterioration can be mass-produced at an improvedyield.

As for the transition temperature (Tt), it is occasionally somewhatdifferent depending upon the constitution of the apparatus used or/andthe film-forming conditions employed. However in general, since thereappears a transition region with respect to hydrogen content of the filmas shown in FIG. 2 when the t_(A) (the period of time during which inertgas plasma is irradiated to the a-Si film formed in the step (a)) ismade to be 30 seconds or more, the concrete value of the Tt isdetermined and the film formation is carried out at a substratetemperature situated in the substrate temperature region exceeding thepredetermined transition temperature. Specifically, the transitiontemperature may be a low temperature of about 250° C., but in general,it is made to be about 300° C.

As previously described, when the thickness of an a-Si film depositedduring the prescribed period of time t_(D) in the step (a) is made to beless than 10 Å, the a-Si film becomes markedly crystallized in the step(b). In order to more clarify this situation, the followingfilm-formation experiments were conducted. That is, (1) there wereprepared a plurality of silicon film samples having a thickness of about1 um by repeating the film-forming cycle comprising the step (a) ofdepositing an a-Si film and the step (b) of irradiating inert gas plasmaunder the film-forming conditions capable of affording a silicon filmcomprised of amorphous phase only, wherein the prescribed period of time(t_(D)) (that is, thickness of the resulting film in other words) waschanged in each case; and (2) there were prepared a plurality of siliconfilm samples having a thickness of about 1 um by repeating thefilm-forming cycle comprising the step (a) of depositing an a-Si filmand the step (b) of irradiating inert gas plasma under the film-formingconditions capable of affording a silicon film comprised of amorphousphase and crystalline phase, wherein the prescribed period of time(t_(D)) (that is, thickness of the resulting film in other words) waschanged in each case.

It is known that it is possible to recognize the crystallinity of adeposited film by observing a signal peak at 520 cm⁻¹ belonging to acrystalline phase and a signal peak at 480 cm⁻¹ belonging to anamorphous phase in a Raman spectrum obtained as for the deposited film.

Therefore, as for each of the resultant silicon film samples obtained inthe above (1) and (2), its crystallinity was examined as to whether thefilm comprises an amorphous phase, crystalline phase or both of said twophases by observing a signal peak at 520 cm⁻¹ resulting from thecrystalline phase and a signal peak at 480 cm⁻¹ resulting from theamorphous phase in a Raman spectrum obtained as for the silicon film.

The observed results obtained were graphically shown in FIG. 3, in whichthe mark "o" concerns the condition under which an a-Si film comprisingamorphous phase only was formed, and the mark "x" concerns the conditionunder which a Si film comprising amorphous phase and crystalline phasewas formed.

From what shown in FIG. 3, it was found that the thickness of an a-Sifilm formed during the t_(D) in the respective repetition step (a) isnecessary to be at least 10 Å in order to make the a-Si film formed ineach repetition cycle to be identical.

It was also found that an a-Si film comprised of amorphous phase only,which is free of crystalline phase, is formed in the respectiverepetition step (a) as long as it is formed under the film-formingconditions capable affording an a-Si film while controlling thethickness of the resulting film to at least 10 Å.

Then, as a result of further experimental studies, the following werefound. That is, as long as the thickness of an a-Si film deposited onthe substrate in the respective repetition step (a) is at least 10 Å,the hydrogen content of the a-Si film can be properly controlled byregulating the t_(A) while maintaining the substrate at a substratetemperature corresponding to or exceeding the transition temperature Ttshown in FIG. 2. Where the t_(A) is properly prolonged, the quality ofthe resulting a-Si film hardly depends upon the substrate temperaturebut also slightly depend upon the film formation period of time; and asa result, there can be stably and repeatedly produced a high qualitya-Si semiconductor film with a reduced hydrogen content which is low inspin density, excels in carrier transportation characteristics, and isfree of light deterioration.

Shown in FIG. 4 are of the interrelation between the t_(A) (the periodof time during which inert gas plasma is irradiated to the a-Si film)and the hydrogen content with respect to the film formed by repeatingthe film formation cycle comprising the step (a) of depositing an a-Sifilm on a substrate and the step (b) of irradiating inert gas plasma tothe a-Si film.

The line a in FIG. 4 is of the results obtained in the case where theabove film formation cycle was repeated to obtain an a-Si film of about1 um in thickness while maintaining the thickness of an a-Si film formedin the respective repetition step (a) at 30 Å and changing the t_(A)(the period of time during which inert gas plasma is irradiated in therespective repetition step (b)).

The line b in FIG. 4 is of the results obtained in the case where theabove film formation cycle was repeated to obtain an a-Si film of about1 um in thickness while maintaining the thickness of an a-Si film formedin the respective repetition step (a) at 150 Å and changing the t_(A)(the period of time during which inert gas plasma is irradiated in therespective repetition step (b)).

As a result of studies based on what shown in FIG. 4, the following werefound. That is, in the case where the thickness of an a-Si film formedin each film formation cycle is made to be as large as more than 100 Å(that is, in the case of the line b in-FIG. 4), the hydrogen content ofthe a-Si film finally obtained is not reduced (this means that nodistinguishable effect is provided by the step (b) of irradiating inertgas plasma), the hydrogen content of the a-Si film formed relativelydepends upon the substrate temperature, and the resulting a-Si film ismore or less around the same as that of an a-Si film formed by theconventional GD method, which is not satisfactory especially withrespect to light deterioration. On the other hand, in the case of theline a in FIG. 4, the hydrogen content of an a-Si film in each filmformation cycle is desirably reduced as long as the substratetemperature upon film formation is maintained at a temperaturecorresponding to or exceeding the transition temperature Tt (see, FIG.2) wherein the said hydrogen content substantially does not depend uponthe substrate temperature, and as a result, there is afforded adesirable a-Si semiconductor film which is free of light deterioration.

In view of the above experimental results, it was found that in order toobtain an a-Si semiconductor film with a reduced hydrogen content whichis free especially of light deterioration by repeating the filmformation cycle comprising the step (a) of depositing an a-Si film on asubstrate and the step (b) of irradiating inert gas plasma to the a-Sifilm, it is desired for the thickness of the a-Si film formed in therespective repetition step (a) to be less than 100Å or preferably, lessthan 50 Å.

Now, the mean deposition rate V_(D) of the a-Si film formed in therespective repetition step (a) depends upon the thickness of the saida-Si film as apparent from the foregoing equation (3). The period oftime t_(A) or t_(A) ' (see, FIG. 1) during which the respectiverepetition step (b) of irradiating inert gas plasma to the a-Si filmformed in the preceding step (a) does not concern the film deposition.In order to increase the deposition rate of the a-Si film, it isdesirable to thicken the a-Si film formed in the respective repetitionstep (a). However, it is difficult to increase the mean deposition rateV_(D) without reducing the property of an a-Si film formed in therespective repetition step (a) in the case where the thickness of thesaid a-Si film is made to be a relatively small value.

It was found through experiments that such difficulty can be effectivelyeliminated in the case of applying a bias voltage to the substrate sideupon performing the respective repetition step (b), wherein a desirablea-Si film having such thickness can be formed without reducing the filmproperty.

Shown in FIG. 5 are of the interrelation between the thickness 1 (=v_(D)×t_(D)) of an a-Si film deposited in the respective repetition step (a)and the hydrogen content with respect to the film formed by repeatingthe film formation cycle comprising the step (a) of depositing an a-Sifilm on a substrate and the step (b) of irradiating inert gas plasma tothe a-Si film while applying a bias voltage to the substrate.

The line a in FIG. 5 is of the results obtained in the case where theabove film formation cycle was repeated to obtain an a-Si semiconductorfilm of about 1 um in thickness while maintaining the substrate at atemperature higher than the Tt (transition temperature)(see, FIG. 2),making the t_(A) (the period of time during which the respectiverepetition step (b) is performed) constant at 20 seconds, changing thet_(D) (the period of time during which the respective repetition step(a) is performed), and applying a bias voltage to the substrate sidewherein the substrate was electrically grounded in the respectiverepetition step (b).

The line b in FIG. 5 is of the results obtained in the case where theabove film formation cycle was repeated to obtain an a-Si semiconductorfilm of about 1 um in thickness while maintaining the substrate at atemperature higher than the Tt (transition temperature)(see, FIG. 2),making the t_(A) (the period of time during which the respectiverepetition step (b) is performed) constant at 20 seconds, changing thet_(D) (the period of time during which the respective repetition step(a) is performed), and applying a bias voltage of -75 V to the substratein the respective repetition step (b).

From what shown in FIG. 5, it was found that the effect of irradiatinginert gas plasma is provided even in the region wherein the i isrelatively large in the case of applying a bias voltage applied uponperforming the respective repetition step (b). And it was also foundthat a desirable a-Si semiconductor film excelling in semiconductorcharacteristics is afforded even in the case of making the thickness ofan a-Si film formed in the respective repetition step (a) to berelatively greater as long as a bias voltage is applied to the substrateupon performing the respective repetition step (b).

The repetition step (a) of depositing an a-Si film on a substrate in thepresent invention may be performed by a plasma CVD method such as RFplasma CVD method (RF glow discharge decomposition method), microwaveplasma CVD method, etc. However, other film-forming methods such aslight-induced CVD method, electron cyclotron resonance plasma CVD method(ECR plasma CVD method), reactive sputtering method using gas containingH₂ gas, etc. may be also employed in order to perform the repetitionstep (a) of depositing an a-Si film.

The repetition step (b) of irradiating inert gas plasma to the a-Sideposited film formed in the repetition step (a) in the presentinvention may be performed by means of a conventional plasma-irradiatingtechnique. The important factor at the time of irradiating inert gasplasma to the deposited film is to supply ions of inert gas (hereinafterreferred to as inert gas ions) and/or excited atoms of inert gas(hereinafter referred to as inert gas atoms) to the surface of the a-Sideposited film, wherein luminescent portion of the inert gas plasma isnot necessary to be in contact with the surface of the a-Si depositedfilm. As the simplest method of irradiating inert gas plasma to the a-Sideposited film, there can be mentioned a method of carrying out highfrequency glow discharging (RF glow discharging) with a capacitivecoupling system or an inductive coupling system under the condition ofabout 100 mtorr for the inner pressure. The repetition step (b) ofirradiating inert gas plasma to the a-Si deposited film by means of theabove glow discharging technique is performed, for example, by placingthe substrate having an a-Si film thereon (said a-Si film having beendeposited in the repetition step (a)) prior to the repetition step (b))in or in the vicinity of glow discharge plasma produced from inert gas,whereby supplying ions and/or excited atoms of the inert gas to thesurface of the a-Si film deposited on the substrate. Other than theseglow discharge plasma irradiating methods, there may be optionallyemployed a method of producing inert gas plasma with the use ofmicrowave energy and diffusing said inert gas plasma to the surface ofthe a-Si film formed on the substrate or an method with the use of anECR technique wherein inert gas plasma is produced and said inert gasplasma is irradiated to the surface of the a-Si film formed on thesubstrate utilizing magnetic field.

In the case of performing the repetition step (a) of depositing an a-Sifilm on the substrate by means of a plasma CVD technique using a plasmaCVD apparatus, it is possible to perform the repetition step (b) ofirradiating inert gas plasma to the a-Si film formed in the step (a) inthe identical plasma CVD apparatus. In this case, a pronounced effect isprovided if a rare gas (inert gas) is used in the repetition step (a) ofdepositing an a-Si film. That is, the repetition step (b) can beproperly performed by simply suspending the introduction of afilm-forming raw material gas such as SiH₄, Si₂ H₆, etc. used in thestep (a) upon performing the step (b); and the successive repetitionstep (a) can be properly performed by simply resuming the introductionof said film-forming raw material gas. In this case, a due care shouldbe made of the ratio between the film-forming raw material gas and therare gas (inert gas) upon performing the repetition step (a). That is,in the case where the ratio of the film-forming raw material gas to therare gas (inert gas) is excessively reduced upon performing therepetition step (a), residual rare gas (inert gas) is often contaminatedinto the resulting a-Si film and as a result, the resulting a-Si filmbecomes such that is poor in film property. In view of this, theconcentration of the film-forming raw material gas upon performing therepetition step (a) is desired to be controlled to preferably 30 % orabove or more preferably, 50 % or above. It is possible to continuouslymaintain the inert gas plasma in the two repetition steps (a) and (b).In this case, deposition of an undesirable a-Si film which is liable tooccur at the beginning stage of causing discharge plasma can effectivelyprevented.

Further, the repetition step (a) of depositing an a-Si film on asubstrate and the repetition step (b) of irradiating inert gas plasma tothe a-Si film can be independently controlled one from the other bydifferentiating the concentration of the rare gas (inert gas) or/and thehigh frequency power applied in each of the repetition steps (a) and(b). In this case, a more desirable annealing effect is provided to thea-Si film to be treated in the repetition step (b).

There is not a particular limitation for the discharging conditions uponperforming the repetition step (b) of irradiating inert gas plasma to ana-Si film. However, in the case where the power applied for causingdischarge is excessively low, sufficient excited atoms are notgenerated, and unless the t_(A) is considerably prolonged, desirableeffects by the irradiation of inert gas plasma are not provided.

The important thing in the repetition step (b) is to sufficiently supplyions and/or excited atoms of the rare gas (inert gas) to the surface ofthe a-Si film deposited on the substrate in the repetition step (a) aspreviously described. This can be attained in the range of theconventional discharge conditions.

In the case of applying the bias voltage to the substrate having an a-Sifilm thereon upon performing the repetition step (b) of irradiatinginert gas plasma to the a-Si film, the bias voltage may be a self biasvoltage of the inert gas plasma. In this case, such self bias voltagemay be applied by floating the substrate from the earth. Other thanthis, the bias voltage may be a bias voltage applied from an externalpower source capable of outputting said bias voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in more detail with reference tothe following examples, which are not intended to restrict the scope ofthe invention.

Example 1

There was prepared a schottky-barrier type image-reading sensor of 161p/mm in element density and 300 mm in length comprising a plurality ofsemiconductor elements each having the constitution shown in FIG. 7,using the fabrication apparatus shown in FIG. 6.

The constituent semiconductor element of the schottky-barrier typeimage-reading sensor shown in FIG. 7 comprises substrate 701, electrodelayer 702 disposed on the substrate 701, semiconductor layer 703disposed on the electrode layer 702 and transparent and conductive layer704 disposed on the semiconductor layer 703. Numeral reference 705stands for a passivation film and numeral reference 706 stands for awiring.

The fabrication apparatus shown in FIG. 6 comprises three film-formingvacuum chambers 600, 601 and 603 being arranged between a substratesupply vacuum chamber 603 and a substrate taking-out vacuum chamber 604.Each of these vacuum chambers is provided with an exhausting pump 611,612, 613, 614 or 615. The substrate supply vacuum chamber 603 is sostructured that a substrate can be introduced therein while maintainingthe vacuum of each of the film-forming vacuum chambers 600, 601 and 602.Similarly, the substrate taking-out vacuum chamber is so structured thata substrate can be taken out while maintaining the vacuum of each of thefilm-forming vacuum chambers 600, 601 and 602.

Each of numeral references 605, 606, 607 and 608 stands for a gate valvefor isolating the adjacent two vacuum chambers.

The fabrication apparatus shown in FIG. 6 is provided with atransportation mechanism (not shown in the figure) capable oftransporting the substrate from one vacuum chamber to the other vacuumchamber.

Each of numeral references 609, 655, 656, 657 and 610 stands for asubstrate holder. Each of the substrate holders 609, 655, 656 and 657,other than the substrate holder 610, is so designed that it serves as anelectrode. And each of the substrate holders 609, 655, 656 and 657 isprovided with an electric heater 616, 617, 618 or 619 which serves toheat the substrate. Numeral reference 631 stands for a reservoircontaining N₂ gas which is connected to the substrate supply vacuumchamber 603. Numeral reference 632 stands for a reservoir containing N₂gas which is connected to the substrate taking-out vacuum chamber 604.The reservoir 631 is used to supply N₂ gas into the substrate supplyvacuum chamber 603 when the vacuum chamber is opened upon introducingthe substrate thereinto. The reservoir 632 is used to supply N₂ gas intothe substrate taking-out vacuum chamber 604 when the vacuum chamber isopened upon taking out the substrate therefrom. Each of numeralreferences 622, 658 and 623 stands for a high frequency electrode.Numeral reference 620 stands for a Cr-target disposed on the highfrequency electrode 622. Numeral reference 621 stands for an ITO-targetdisposed on the high frequency electrode 623. Each of numeral references624, 625 and 626 stands for a matching box. Each of numeral references628, 629 and 630 stands for a high frequency power source of 13.56 MHz.Each of numeral references 627, 636, 637, 638, 639, 640, 641, 642, 643,644, 652, 653, 658, 659, 660 and 661 stands for a valve for the gaspipying. Numeral reference 634 stands for a three-way valve capable ofswitching the route to the film-forming vacuum chamber 601 from theroute to a gas line exhaust pump 635 serving to stabilize the gas flowrate. Each of numeral references 645, 646, 647 and 654 stands for a massflow controller. Numeral reference 648 stands for a reservoir containingAr gas. Numeral reference 649 stands for a reservoir containing H₂ gas.Numeral reference 650 stands for a reservoir containing SiH₄ gas.Numeral reference 651 stands for a reservoir containing O₂ gas.

The shottity-barrier type image-reading sensor was prepared using theabove fabrication apparatus as follows.

A well-cleaned glass plate (Corning glass plate No. 7059 produced byCorning Glass Company) as the substrate 701 was introduced into thesubstrate supply vacuum chamber 603, and the chamber was evacuated to apredetermined vacuum degree. The electric heater 616 was switched on toheat the substrate to 150° C., followed by transferring the substrateinto the film-forming vacuum chamber 600, wherein a Cr film as theelectrode layer 702 was formed on the substrate (glass plate) bysputtering the Cr-target 620 in a gas atmosphere comprising Ar gassupplied from the gas reservoir 648 by opening the valves 638, 641 and642 while regulating the inner pressure to a predetermined value. Thenthe substrate having the Cr film as the electrode layer 702 thereon wastransferred into the film-forming vacuum chamber, wherein an about 6000Å thick a-Si film as the semiconductor layer 703 was formed on thepreviously formed Cr electrode layer 702 by practicing the foregoingfilm-forming process of the present invention, comprising repeating thefilm-forming cycle comprising the step (a) of depositing an a-Si filmusing SiH₄ gas and Ar gas and the step (b) of irradiating plasma of Argas (inert gas) to the a-Si film. The film-forming cycle was repeated200 times. The respective repetition step (a) was performed for 10seconds as for the t_(D) by causing glow discharge in a gas atmospherecomprising SiH₄ gas and Ar gas while introducing SiH₄ gas from thereservoir 650 at a flow rate of 20 sccm and Ar gas from the reservoir648 at a flow rate of 10 sccm and applying high frequency power of 20 Wunder the conditions of 350° C. (which is higher than the transitiontemperature which was found to be 300° C. in this case) as for thesubstrate temperature and 0.1 Torr as for the inner pressure, to therebyan a-Si film of 30 Å in mean thickness. The respective repetition step(b) was performed for 30 seconds as for the t_(A) by causing glowdischarge in a gas atmosphere comprising Ar gas while introducing Ar gasfrom the reservoir 648 at a flow rate of 10 sccm and applying highfrequency power of 20 W under the conditions of 350° C. (which is higherthan the transition temperature of 300° C.) as for the substratetemperature and 0.1 Torr as for the inner pressure, to thereby irradiateAr gas plasma to the a-Si film formed in the antecedent repetition step(a). Thereafter, the substrate was transferred into the film-formingvacuum chamber 602 wherein an ITO film as the transparent and conductivelayer 704 was formed on the previously formed a-Si semiconductor layer703 by sputtering the ITO-target in a gas atmosphere comprising Ar gasand O₂ gas while maintaining the substrate at a temperature of 150° C.and while introducing Ar gas from the reservoir 648 and O₂ gas from thereservoir 651 at the respective predetermined flow rates. The resultantwas then transferred into the substrate taking-out vacuum chamber 604,and after closing the gate valve 608, the resultant was cooled and takenout from the substrate taking-out vacuum chamber. The resultant wassubjected to patterning by means of photolithography. The patternedsample thus obtained was wired to a driving circuit by way of bonding tothereby obtain a schottky-barrier type image-reading sensor.

In the above, prior to forming the a-Si layer in the film-forming vacuumchamber 601, the temperature distribution of the substrate having beenheated to 350° C. was examined. As a result, there was a temperaturedifference of 58° C. between the temperature at the side end portion andthat at the central portion of the substrate.

As for the resultant image-reading sensor, sensitivity, dark current,residual image and reduction in sensitivity by light irradiation wereevaluated with respect to each of the constituent elements situated atthe opposite side positions and the constituent element situated at thecentral position. As a result, no distinguishable difference was foundamong these elements observed with respect to any of the evaluationitems. And no distinguishable reduction was observed as for thesensitivity even after the irradiation of light having a strongintensity.

Independently, the above procedures were repeated, except that theformation of the a-Si film in the film-forming vacuum chamber 601 wascarried out in a conventional manner without performing the foregoingfilm formation cycle and while maintaining the substrate at atemperature of 250° C., to thereby obtain a comparative schottky-barriertype image-reading sensor.

In the above, prior to forming the a-Si layer in the film-forming vacuumchamber 601, the temperature distribution of the substrate having beenheated to 250° C. was examined. As a result, there was a temperaturedifference of 30° C. between the temperature at the side end portionsand that at the central portion of the substrate.

As for the resultant comparative image-reading sensor, sensitivity, darkcurrent, and residual image were evaluated with respect to each of theconstituent elements situated at the opposite side positions and theconstituent element situated at the central position. As a result, therewere found a distinguishable variation of 10% for the sensitivity, 15%for the dark current, and 23% for the residual image among theseconstituent elements observed. And each of the constituent elements wasexposed to light having a strong intensity in order to evaluate thesituation of reduction in the sensitivity. As a result, there wasobserved a distinguishable deterioration in the sensitivity as for eachof the constituent elements.

Example 2 and Comparative Examples 1 to 3

There was prepared a pin junction photovoltaic element of theconfiguration shown in FIG. 8 in each of Example 2 and ComparativeExamples 1 to 3.

In FIG. 8, numeral reference 800 stands for the entire of a pin junctionphotovoltaic element. The pin junction photovoltaic element 800 shown inFIG. 8 comprises a substrate 801, a lower electrode 802 disposed on thesubstrate, an n-type semiconductor layer 803 disposed on the lowerelectrode, an i-type semiconductor layer 804 disposed on the n-typesemiconductor layer, a p-type semiconductor layer 805 disposed on then-type semiconductor layer, an upper electrode 806 disposed on thep-type semiconductor layer and a collecting electrode 807 disposed onthe upper electrode.

Example 2

There was prepared a pin junction photovoltaic element of theconfiguration shown in FIG. 8, using the plasma CVD apparatus shown inFIG. 9. The plasma CVD apparatus shown in FIG. 9 comprises aconventional plasma CVD apparatus provided with an additional gasexhaust system which serves to stabilize gas flow in the gas pipe line.

In FIG. 9, numeral reference 900 stands for a reaction chamber, andnumeral reference 901 stands for a substrate placed on a substrateholder 903 which serves also as an anode electrode. The substrate holder903 is electrically grounded through wiring 905. The substrate holder903 is provided with an electric heater 904. Numeral reference 902stands for a cathode electrode which is extending from a RF power sourceof 13.56 MHz through a matching box 906. The reaction chamber 900 isprovided with an exhaust pipe 908 connected to an exhaust pump 909. Eachof numeral references 910, 912 and 913 stands for a gas supply pipe.Numeral reference 916 stands for a three-way valve disposed between thegas supply pipe 912 and the gas supply pipe 913. The three-way valve 916is connected through an exhaust pipe 914 to an exhaust pump 915. Each ofnumeral references 922, 932, 942, 952, 962 and 972 stands for an inletvalve for a raw material gas from a gas reservoir (not shown). Each ofnumeral references 921, 931, 941, 951, 961 and 971 stands for a massflow controller. Each of numeral references 920, 930, 940, 950, 960 and970 stands for an exit valve.

In the above plasma CVD apparatus, the gas pipe way can be connected toeither the gas supply pipe 913 or the exhaust pipe 914 by switching thethree-way valve 916. When the gas pipe way is connected to the gassupply pipe 913 by switching the three-way valve 916, a raw material gassupplied through the gas pipe way 912 is introduced into the reactionchamber 900. On the other hand, when the gas pipe way is connected tothe exhaust pipe 914 by switching the three-way valve 916, a rawmaterial gas supplied through the gas pipe way 912 is exhausted outsidethe reaction chamber 900 through the exhaust pipe 914 by the action ofthe exhaust pump 915. In the case where the gas pipe way is connected toeither the gas supply pipe 913 or the exhaust pipe 914 by switching thethree-way valve 916, the mass flow controller 961 normally works sincethe gas pressure in the pipe way 912 is lower than the atmosphericpressure. Further, in any case, no additional gas residue is occurred inthe gas pipe way and because of this, the introduction of a raw materialgas into the reaction chamber 900 at a predetermined flow rate issecured when the gas pipe way is connected to the gas supply pipe 913 byswitching the three-way valve 916.

The pin junction photovoltaic element of the configuration shown in FIG.8 was prepared using the above plasma CVD apparatus as follows.

Firstly, there was provided a well-cleaned stainless steel plate of SUS304 (trademark name) of 5 cm×5 cm in size and having a leveled surfaceof 0.05 μm in Rmax as the substrate 801.

The stainless steel plate as the substrate 801 was placed in aconventional sputtering apparatus (not shown) and the inside of thesputtering apparatus was evacuated to about 10⁻⁷ Torr. Then, Ar gas wasintroduced into the sputtering apparatus, and the inner pressure wascontrolled to 5 mtorr. DC plasma was caused with a power of 200 W tosputter an Ag-target, whereby forming an about 5000 Å thick Ag film onthe substrate. Successively, using the same sputtering apparatus, therewas formed an about 5000 Å thick ZnO film on the Ag film. That is, theabove procedures of forming the Ag film were repeated, except that theAg-target was replaced by a ZnO-target, to thereby form said ZnO film onthe Ag film. Thus, there was formed the lower electrode layer 802 on thestainless steel plate as the substrate 801.

The substrate 801 was introduced into the reaction chamber 900 of theplasma CVD apparatus shown in FIG. 9 and it was placed on the substrateholder 903. The inside of the reaction chamber 900 was evacuated to avacuum of about 10⁻⁶ Torr by driving the exhaust pump 909. The electricheater 904 was actuated to heat the substrate 801 (the substrate 901 inFIG. 9) to 350° C., and the substrate was maintained at thistemperature. The exit valve 920 and the inlet valve 922 were opened tointroduce SiH₄ gas from a SiH₄ gas reservoir (not shown) through the gassupply pipe 910 into the reaction chamber 900 at a flow rate of 30 sccmwhile regulating the flow rate by the mass flow controller 921. At thesame time, the exit valve 930 and the inlet valve 932 were opened tointroduce H₂ gas from a H₂ gas reservoir (not shown) through the gassupply pipe 910 into the reaction chamber 900 at a flow rate of 30 sccmwhile regulating the flow rate by the mass flow controller 931. Further,the exit valve 950 and the inlet valve 952 were opened to introduce PH₃gas (diluted with H₂ gas to 5%) (hereinafter referred to as PH₃ /H₂ gas)from a PH₃ /H₂ gas reservoir (not shown) through the gas supply pipe 910into the reaction chamber 900 at a flow rate of 10 sccm while regulatingthe flow rate by the mass flow controller 951.

The inner pressure of the reaction chamber 900 was controlled to 1.5Torr by the action of the exhaust pump 909. Then, the RF power source907 was switched on to apply a RF power of 10 W through the matching box906 between the anode electrode 903 (the substrate holder) and thecathode electrode 902, to thereby cause plasma discharge for 3 minutes,whereby forming a 400 Å thick n-typ a-Si:H:P film to be the n-typesemiconductor layer 803 on the lower electrode layer 802.

After the formation of the n-type semiconductor layer 803 having beencompleted, the application of the RF power and the introduction of thethree gases were terminated. The inside of the reaction chamber 900 wasevacuated to a vacuum of about 10⁻⁶ Torr.

The substrate was maintained at 350° C. Then, the exit valve 970 and theinlet valve 972 were opened to introduce He gas from a He gas reservoir(not shown) through the gas supply pipe 910 into the reaction chamber900 at a flow rate of 20 sccm while regulating the flow rate by the massflow controller 971. Concurrently, after the pipe way 912 having beenconnected to the exhaust pipe 914 by switching the three-way valve 916,the exit valve 960 and the inlet valve 962 were open to flow SiH₄ gasinto the pipe way 912 from a SiH₄ gas reservoir (not shown) at a flowrate of 30 sccm. Maintaining this state, the RF power source 907 wasswitched on to apply a RF power of 20 W between the anode electrode 903(the substrate holder) and the cathode electrode 902 to cause plasmadischarge generating plasma of the He gas (that is, He gas plasma)between the anode electrode 903 (the substrate holder) and the cathodeelectrode 902. After confirming that said He gas plasma was generated,the pipe way 912 was connected to the gas supply pipe 913 by switchingthe three-way valve 916, to thereby introduce SiH₄ gas into the reactionchamber 900 at a flow rate of 30 sccm for 30 seconds. Then, the pipe way912 was connected to the exhaust pipe 914 by switching the three-wayvalve 916, to thereby suspend the introduction of the SiH₄ gas into thereaction chamber 900. After this state having been maintained for 40seconds, the pipe way 912 was connected to the gas supply pipe 913 byswitching the three-way valve 916, to thereby resume the introduction ofthe SiH₄ gas into the reaction chamber 900 at a flow rate of 30 sccm.After this state having been maintained for 10 seconds, the pipe way 912was connected to the exhaust pipe 914 by switching the three-way valve916, to thereby suspend the introduction of the SiH₄ gas into thereaction chamber 900 for 40 seconds. The repetition of performingalternately the step of introducing the SiH₄ gas into the reactionchamber 900 for 10 seconds and the step of suspending the introductionof the SiH₄ gas into the reaction chamber 900 for 40 seconds wasconducted 200 times. As a result, there was formed a 6000 Å thick i-typea-Si:H semiconductor film to be the n-type semiconductor layer 804 onthe previously formed n-type a-Si:H:P film as the n-type semiconductorlayer 803.

After the formation of the i-type semiconductor layer 804 having beencompleted, the application of the RF power and the introduction of thethree gases were terminated.

The inside of the reaction chamber 900 was evacuated to a vacuum ofabout 10⁻⁶ Torr. The substrate was maintained at 200° C. The exit valve920 and the inlet valve 922 were opened to introduce SiH₄ gas from aSiH₄ gas reservoir (not shown) through the gas supply pipe 910 into thereaction chamber 900 at a flow rate of 1 sccm while regulating the flowrate by the mass flow controller 921. At the same time, the exit valve930 and the inlet valve 932 were opened to introduce H₂ gas from a H₂gas reservoir (not shown) through the gas supply pipe 910 into thereaction chamber 900 at a flow rate of 300 sccm while regulating theflow rate by the mass flow controller 931. Further, the exit valve 940and the inlet valve 942 were opened to introduce B₂ H₆ gas (diluted withH₂ gas to 5%) (hereinafter referred to as B₂ H₆ /H₂ gas) from a B₂ H₆/H₂ gas reservoir (not shown) through the gas supply pipe 910 into thereaction chamber 900 at a flow rate of 10 sccm while regulating the flowrate by the mass flow controller 941.

The inner pressure of the reaction chamber 900 was controlled to 1.5Torr by the action of the exhaust pump 909. Then, the RF power source907 was switched on to apply a RF power of 200 W through the matchingbox 906 between the anode electrode 903 (the substrate holder) and thecathode electrode 902, to thereby cause plasma discharge for 5 minutes,whereby forming a 100 Å thick p-typ microcrystalline SI:H:B film to bethe p-type semiconductor layer 805 on the previously formed i-typesemiconductor layer 804. (As for the SI:H:B film, it was confirmed to bemicrocrystalline by separately forming a silicon film under the sameconditions under which the said p-type semiconductor film and examiningthe crystallinity thereof by RHEED.)

The resultant obtained in the above was taken out from the reactionchamber 900 and it was transferred into a conventional resistanceheating evaporation apparatus. The inside of the resistance heatingevaporation apparatus was evacuated to about 10⁻⁷ Torr. Then, thesubstrate was maintained at 160° C. O₂ gas was introduced thereinto, andthe inner pressure was controlled to 0.5 mtorr. An alloy composed of Inand Sn was evaporated by means of resistance heating, to thereby form a700 Å thick ITO film to be the upper electrode 806 capable of servingalso to provide reflection preventive effects on the previously formedp-type semiconductor layer 805. The resultant thus obtained wastransferred into a conventional dry etching apparatus, wherein it wasdivided to obtain 25 element samples of 1 cm×1 cm in size. As for eachof the resultant samples, an aluminium collecting electrode was formedon the upper electrode 806 by means of a conventional electron beamevaporation technique. Thus, there were obtained 25 pin junctionphotovoltaic element samples (Sample No. 1).

Comparative Example 1

The procedures of Example 2 were repeated, except that the formation ofthe i-type semiconductor layer 804 was carried out by a conventionalmanner wherein SiH₄ gas was introduced through through the gas supplypipe 910 into the reaction chamber 900 at a flow rate of 30 sccm and RFglow discharge was caused for 200 seconds in the same manner as in thecase of forming the n-type semiconductor layer in Example 2, to therebyobtain 25 comparative pin junction photovoltaic element samples (SampleNo. 2).

Comparative Example 2

The procedures of Example 2 were repeated, except that the substratetemperature upon forming the i-type semiconductor layer 804 was changedfrom 350° C. to 250° C., to thereby obtain 25 comparative pin junctionphotovoltaic element samples (Sample No. 3).

Comparative Example 3

The procedures of Comparative Example 2 were repeated, except that thesubstrate temperature upon forming the i-type semiconductor layer 804was changed from 350° C. to 250° C., to thereby obtain 25 comparativepin junction photovoltaic element samples (Sample No. 4).

Evaluation

As for each of Samples Nos. 1 to 4 obtained in Example 2 and ComparativeExamples 1 to 3 each comprising 25 photovoltaic element samples, initialphotoelectric conversion efficiency, deterioration ratio thereof, andhydrogen content of the i-type semiconductor layer were evaluatedrespectively in the following evaluation manner. Evaluation of theinitial photoelectric conversion efficiency

A voltage-current characteristic curve is obtained on the basis of theresults obtained by subjecting the pin photovoltaic element sample toirradiation of pseudo sun light of AM-1.5 with 100 mw/cm² in intensityby using a solar cell simulator. And an initial photoelectric conversionefficiency η(0) is obtained based on the resultant voltage-currentcharacteristic curve. Evaluation of the ratio of deterioration withrespect to the initial photoelectric conversion efficiency.

On the basis of the value of open-circuit voltage and the value ofshort-circuit current obtained at the time of obtaining the abovevoltage-current characteristic curve, an optimum load is calculated. Aprescribed load resistance is connected to the photovoltaic elementsample. The photovoltaic element sample to which the prescribed loadresistance being connected is placed on a sample table being maintainedat 25° C., and it is subject to irradiation of pseudo sun light ofAM-1.5 with 100 mw/cm² in intensity continuously for 500 hours. And aphotoelectric conversion efficiency η (500) after the irradiation ofpseudo sun light of AM-1.5 with 100 mw/cm² in intensity continuously for500 hours is obtained in the same manner as in the case of obtaining theinitial photoelectric conversion efficiency. On the basis of the valueof the foregoing η(0) and the η(500), the ratio of deterioration(1-η(500)/η(0)) is obtained. Evaluation of the hydrogen content of thei-type semiconductor layer

The hydrogen content of the i-type semiconductor layer is measured bySIMS.

As for the evaluated result of each of the initial photoelectricconversion efficiency η(0), the ratio of deterioration in photoelectricconversion efficiency and the hydrogen content of the i-typesemiconductor layer with respect to the 25 photovoltaic element samplesof each of Samples Nos. 1 to 4, a mean value was obtained among the 25photovoltaic element samples.

The evaluated results obtained were collectively shown in Table 1, inwhich the evaluated value of the initial photoelectric conversionefficiency and the evaluated value of the ratio of deterioration inphotoelectric conversion efficiency with respect each of Samples Nos. 1to 3 are values relative to those with respect to Sample No. 4 which aremade to be 1 respectively.

As the results of Table 1 illustrate, it is understood that the pinjunction photovoltaic element obtained in Example 2 of the presentinvention is surpassing any of the comparative pin junction photovoltaicelements obtained in Comparative Examples 1 to 3 with respect to any ofthe evaluation items. Particularly, the pin junction photovoltaicelement obtained in Example 2 of the present invention is markedlysurpassing any of the comparative pin junction photovoltaic elementsobtained in Comparative Examples 1 to 3 with respect to the ratio ofdeterioration in photoelectric conversion efficiency.

Separately, the procedures of each of Example 2 and Comparative Examples1 to 3 for forming the i-type semiconductor layer were repeated to asilicon film on a glass plate to be used for Raman spectroscopicanalysis. Each of the resultant silicon film samples was subjected toRaman spectroscopic analysis. As a result, there was observed a specificbroad peak near 480 cm⁻¹ indicating amorphous silicon film in each case.

Example 3

There was prepared a field effect thin film transistor of theconfiguration shown in FIG. 10, using the plasma CVD apparatus shown inFIG. 9.

The field effect thin film transistor shown in FIG. 10 comprises aninsulating substrate 1001, a gate electrode 1002 in a desired patternformed on the insulating substrate, an insulating layer 103, asemiconductor layer 1004, ohmic contact layers 1005' and 1005", and mainelectrodes 1006' and 1006".

The field effect thin film transistor was prepared as follows.

Firstly, on a well-cleaned glass plate (Corning glass plate No. 7059produced by Corning Glass Company) as the substrate 1001 was formed analuminum film to be the gate electrode 1002 by means of a conventionalevaporation technique. The-substrate was introduced into the reactionchamber 900 of the plasma CVD apparatus shown in FIG. 9 and it wasplaced on the substrate holder 903 capable of serving also as the anodeelectrode as shown by the numeral reference 901 in FIG. 9. The inside ofthe reaction chamber 900 was evacuated to a vacuum of about 10⁻⁶ Torr.The electric heater 904 was actuated to heat the substrate to 350° C.,and the substrate was maintained at this temperature. Then, NH₃ gas, H₂gas and Ar gas were introduced into the reaction chamber 900 through thegas supply pipe 910 at the respective flow rates of 150 sccm, 10 sccmand 10 sccm. The inner pressure of the reaction chamber 900 wascontrolled to and maintained at 0.4 Torr by the action of the exhaustpump 909. The RF power source was switched on to apply a RF power of 10W between the anode electrode 903 (the substrate holder) and the cathodeelectrode 902 through the matching box 906 to cause plasma of a mixtureof NH₃ gas, H₂ gas and Ar gas, wherein the step of introducing Si₂ H₆gas through the gas supply pipe 913 into the reaction chamber 900 at aflow rate of 20 sccm for 20 seconds while connecting the pipe way 912 tothe gas supply pipe 913 by switching the three-way valve 916 and thestep of suspending the introduction of said Si₂ H₆ gas into the reactionchamber 900 for 40 seconds while connecting the pipe way 912 to theexhaust pipe 914 by switching the three-way valve 916 were alternatelyperformed 100 times, whereby forming an about 1500 Å thick a-SiN:H filmto be the insulating layer 103 on the substrate 1001 having the aluminumgate electrode 1002 thereon.

After the formation of the insulating layer 1003 having been completed,the application of the RF power and the introduction of the raw materialgases were terminated. The inside of the reaction chamber 900 wasevacuated to a vacuum of about 10⁻⁶ Torr. The substrate was maintainedat 350° C. Then, Ar gas was introduced through the gas supply pipe 910into the reaction chamber 900 at a flow rate of 10 sccm. The innerpressure of the reaction chamber 900 was controlled to and maintained at0.3 Torr by regulating the exhaust pump 909. The RF power source 907 waswitched on to apply a RF power of 10 W between the anode electrode 903(the substrate holder) and the cathode electrode 902 to cause plasma ofAr gas, wherein the step of introducing Si₂ H₆ gas through the gassupply pipe 913 into the reaction chamber 900 at a flow rate of 20 sccmfor 5 seconds while connecting the pipe way 912 to the gas supply pipe913 by switching the three-way valve 916 and while applying a RF power 3W between the anode 903 (the substrate holder) and the cathode 902 andthe step of suspending the introduction of said Si₂ H₆ gas into thereaction chamber 900 for 40 seconds while connecting the pipe way 912 tothe exhaust pipe 914 by switching the three-way valve 916 and whileapplying a RF power 10 W between the anode 903 (the substrate holder)and the cathode 902 were alternately performed 25 times, whereby formingan about 50 Å thick non-doped a-Si:H film to be the semiconductor layer1004 on the previously formed insulating layer 1003.

After the formation of the semiconductor layer 1004 having beencompleted, the application of the RF power and the introduction of theraw material gases were terminated. The inside of the reaction chamber900 was evacuated to a vacuum of about 10⁻⁶ Torr. The substrate wasmaintained at 350° C. Then, Ar gas was introduced through the gas supplypipe 910 into the reaction chamber 900 at a flow rate of 30 sccm. Theinner pressure of the reaction chamber 900 was controlled to andmaintained at 0.3 Torr by regulating the exhaust pump 909. The RF powersource 907 was witched on to apply a RF power of 10 W between the anodeelectrode 903 (the substrate holder) and the cathode electrode 902 tocause plasma of Ar gas, wherein the step of introducing Si₂ H₆ gascontaining 1000 ppm of PH₃ through the gas supply pipe 913 into thereaction chamber 900 at a flow rate of 20 sccm for 5 seconds whileconnecting the pipe way 912 to the gas supply pipe 913 by switching thethree-way valve 916 and while applying a RF power of 10 W between theanode 903 (the substrate holder) and the cathode 902 and the step ofsuspending the introduction of said Si₂ H₆ gas into the reaction chamber900 for 40 seconds while connecting the pipe way 912 to the exhaust pipe914 by switching the three-way valve 916 and while applying a RF power10 W between the anode 903 (the substrate holder) and the cathode 902were alternately performed 17 times, whereby forming an about 500 Åthick n⁺ -type a-Si:H:P film to be the ohmic contact layer 1005 on thepreviously formed semiconductor layer 1004.

After the formation of the n⁺ -type a-Si:H:P film having been completed,the introduction of the raw material gases and the application of the RFpower were terminated, and the electric heater was switched off. Then,the resultant was taken out from the reaction chamber. On the layercomprising the n⁺ -type a-Si:H:P film was formed an aluminium film to bethe main electrode 106 by means of a conventional evaporation technique.The resultant was subjected to patterning by photolithography to removethe channel-forming portion of each of the n⁺ -type a-Si:H:P film andthe aluminum film, whereby forming the ohmic contact layers 1005' and1005" and the main electrodes 1006' and 1006" with W/L=100, with W beinga channel width and L being a channel length.

Thus, there was obtained a field effect thin film transistor of theconfiguration shown in FIG. 10.

Separately, the above procedures of forming the field effect thin filmtransistor were repeated, except that the formation of the respectivesilicon-containing films was carried out without performing the step ofsuspending the introduction of Si₂ H₆ gas, to thereby obtain acomparative field effect thin film transistor.

As for each of the two field effect thin film transistors, thetransistor characteristics were evaluated by a conventional evaluationmethod. As a result, it was found that the field effect thin filmtransistor obtained in the example of the present invention issurpassing the comparative field effect thin film transistor by morethan one figure with respect to the moving bed. Further, the shift ofVth upon application of a bias voltage was reduced to less than 1/10 inthe case of the field effect thin film transistor obtained in theexample of the present invention. In consequence, it was found that thefield effect thin film transistor obtained in the example of the presentinvention provides improved transistor characteristics which aresuperior to those provided by the comparative field effect thin filmtransistor.

Example 4

There were prepared a plurality of a-Si film samples in the mannersimilar to that in Example 1 for forming the a-Si semiconductor film.

(1) The procedures of Example 1 for forming the a-Si semiconductor filmwere repeated, except that the film-forming conditions were changed to320° C. as for the substrate temperature, 0.1 Torr as for the innerpressure, 20 W as for the RF power applied, 10 seconds as for therepetition step (a), 40 seconds as for the repetition step (b), 20 sccmas for the flow rate of the SiH₄ gas, 10 sccm as for the flow rate ofthe Ar gas, and the repetition step (a) and the repetition step (b) wererepeated alternately 300 times, to thereby form an about 9000 Å thicka-Si film on a well cleaned glass plate (hereinafter referred to as a-Sifilm sample A).

(2) The procedures of Example 1 for forming the a-Si semiconductor filmwere repeated, except that the film-forming conditions were changed to370° C. as for the substrate temperature, 0.1 Torr as for the innerpressure, 20 W as for the RF power applied, 10 seconds as for therepetition step (a), 40 seconds as for the repetition step (b), 20 sccmas for the flow rate of the SiH₄ gas, 10 sccm as for the flow rate ofthe Ar gas, and the repetition step (a) and the repetition step (b) wererepeated alternately 300 times, to thereby form an about 9000 Å thicka-Si film on a well cleaned glass plate (hereinafter referred to as a-Sifilm sample B).

As for each of the a-Si film samples A and B, the photoelectricconductivity at the side end portions and that at the central portion bydisposing an Al comb-shaped electrode of 250 μm in gap width on each ofthose portions by means of a conventional electron beam evaporationtechnique and measuring the photoconductivity at each of those portions.

As a result, there was not a distinguishable difference with respect tothe photoconductivity among those portions examined as for each of thea-Si film samples A and B. And each of the a-Si film samples A and Bexhibited a photoconductivity of 10⁻⁴ to 10⁻⁵ s/cm, which means thateach of the a-Si film samples A and B excels in the film property.

Further, it was found that each of the a-Si film samples A and B is freeof light deterioration.

In addition, as for each of the a-Si film samples A and B, thecrystallinity was examined by Raman spectroscopic analysis. As a result,it was found that each of the a-Si film samples A and B has a specificpeak only near 480 cm⁻¹ but not near 520 cm⁻¹.

(3) The film-forming procedures of the above (1) were repeated, exceptthat the formation of the a-Si film was performed on each of Si-wafersof 1 inch in diameter disposed at the central position and the side endpositions of the glass plate as the substrate, to thereby obtain aplurality of a-Si film samples formed on the respective Si-wafers. Asfor each of the resultant a-Si film samples, the hydrogen content wasexamined by SIMS. As a result, it was found that each of the resultanta-Si film samples contains about 5 atomic %.

(4) The film-forming procedures of the above (2) were repeated, exceptthat the formation of the a-Si film was performed on each of Si-wafersof 1 inch in diameter disposed at the central position and the side endpositions of the glass plate as the substrate, to thereby obtain aplurality of a-Si film samples formed on the respective Si-wafers. Asfor each of the resultant a-Si film samples, the hydrogen content wasexamined by SIMS. As a result, it was found that each of the resultanta-Si film samples contains about 5 atomic %.

For comparison purposes, there were prepared a plurality of comparativea-Si film samples, and the hydrogen content of each of the resultantcomparative a-Si film samples as will be described in the following.

(i) The procedures of the above (1) were repeated, except that theformation of the a-Si film was carried out without performing the stepof suspending the introduction of the SiH₄ gas, to thereby form an about9000 Å thick a-Si film on a well cleaned glass plate (hereinafterreferred to as comparative a-Si film sample A').

(ii) The procedures of the above (2) were repeated, except that theformation of the a-Si film was carried out without performing the stepof suspending the introduction of the SiH₄ gas, to thereby form an about9000 Å thick a-Si film on a well cleaned glass plate (hereinafterreferred to as comparative a-Si film sample B').

As for each of the comparative a-Si film samples A' and B', thephotoconductivity at the side end portions and that at the centralportion by disposing an Al comb-shaped electrode of 250 μm in gap widthon each of those portions by means of a conventional electron beamevaporation technique and measuring the photoconductivity at each ofthose portions.

As a result, there was a distinguishable difference with respect to thephotoconductivity among those portions examined as for each of thecomparative a-Si film samples A' and B'. And each of the comparativea-Si film samples A' and B' did not exhibit a satisfactoryphotoconductivity.

Further, it was found that each of the comparative a-Si film samples A'and B' is accompanied by distinguishable light deterioration.

(iii) The film-forming procedures of the above (i) were repeated, exceptthat the formation of the a-Si film was performed on each of Si-wafersof 1 inch in diameter disposed at the central position and the side endpositions of the glass plate as the substrate, to thereby obtain aplurality of comparative a-Si film samples formed on the respectiveSi-wafers. As for each of the resultant comparative a-Si film samples,the hydrogen content was examined by SIMS. As a result, it was foundthat each of the resultant comparative a-Si film samples contains about11 atomic

(iv) The film-forming procedures of the above (ii) were repeated, exceptthat the formation of the a-Si film was performed on each of Si-wafersof 1 inch in diameter disposed at the central position and the side endpositions of the glass plate as the substrate, to thereby obtain aplurality of comparative a-Si film samples formed on the respectiveSi-wafers. As for each of the resultant comparative a-Si film samples,the hydrogen content was examined by SIMS. As a result, it was foundthat each of the resultant comparative a-Si film samples contains about7 atomic %.

(v) The procedures of the above (1) were repeated, except that theperiod of time (that is, the t_(D)) during which the repetition step (a)is to be performed was changed to 2 seconds, to thereby form an about9000 Å thick comparative a-Si film on a glass plate (hereinafterreferred to as comparative a-Si sample C).

(vi) The procedures of the above (2) were repeated, except that theperiod of time (that is, the t_(D)) during which the repetition step (a)is to be performed was changed to 2 seconds, to thereby form an about9000 Å thick comparative a-Si film on a glass plate (hereinafterreferred to as comparative a-Si sample D).

As for each of the comparative a-Si film samples C and D, thecrystallinity was examined by Raman spectroscopic analysis. As a result,it was found that each of the comparative a-Si film samples C and D hasa strong peak only near 520 cm⁻¹ but an undistinguishable peak near 480cm⁻¹. From this, it is considered that each of the comparative a-Si filmsamples C and D has been crystallized.

Example 5

(1) The procedures of Example 1 for forming the a-Si semiconductor filmwere repeated, except that the film-forming conditions were changed to350° C. (higher than the transition temperature) as for the substratetemperature, 0.1 Torr as for the inner pressure, 20 W as for the RFpower applied, 20 seconds as for the repetition step (a), 40 seconds asfor the repetition step (b), 20 sccm as for the flow rate of the SiH₄gas, 10 sccm as for the flow rate of the Ar gas, and the repetition step(a) and the repetition step (b) were repeated alternately 200 timeswhile applying a bias voltage of -60 V to the substrate, to thereby forma 1.1 μm thick a-Si film on a glass plate and a Si-wafer (hereinafterreferred to as a-Si film sample A).

(2) The procedures of the above (1) were repeated, except no biasvoltage was applied to the substrate, to thereby form a 1.1 μm thicka-Si film on a glass plate and a Si-wafer (hereinafter referred to asa-Si film sample B).

As for each of the a-Si film samples A and B, the photoconductivity andthe hydrogen content were examined.

As a result, it was found that the a-Si film A sample contains 5.5atomic % of hydrogen atoms and exhibits an excellent photoconductivityand on the other hand, the a-Si film sample B contains 8 atomic % ofhydrogen atoms which is lower than that of the a-Si film sample andexhibits a photoconductivity which is lower as much as 1/3 over that ofthe a-Si film sample A. Further, as a result of examining lightdeterioration, it was found that the a-Si film sample A is markedlysurpassing the a-Si film sample B.

Comparative Example 4

The procedures of Example 1 for forming the a-Si semiconductor film wererepeated, except that the film-forming conditions were changed to 250°C. (lower than the transition temperature) as for the substratetemperature, 0.1 Torr as for the inner pressure, 20 W as for the RFpower applied, 20 seconds as for the repetition step (a), 40 seconds asfor the repetition step (b), 20 sccm as for the flow rate of the SiH₄gas, 10 sccm as for the flow rate of the Ar gas, and the repetition step(a) and the repetition step (b) were repeated alternately 200 timeswhile applying a bias voltage of -60 V to the substrate, to thereby forma 1.1 μm thick a-Si film on a glass plate and a Si-wafer.

The resultant a-Si film was examined with respect to the hydrogencontent by SIMS. As a result, it was found that the hydrogen content isvaried in the range of 8 to 13 atomic % between the side end portionsand the central portion. And as a result of examining thephotoconductivity, it was found that the photoconductivity is 10⁻⁶ to10⁻⁷ s/cm, which is not satisfactory. Further, as a result of examininglight deterioration, it was found that the a-Si film is accompanied bydistinguishable light deterioration.

Example 6

There were prepared a plurality of a-Si film samples by repeating theprocedures of Example 4-(1), except that the Ar gas was replaced by Hegas, Ne gas, Kr gas or Xe gas, to thereby form an about 9000 Å thicka-Si film on a well cleaned glass plate in each case.

With respect to each of the resultant a-Si films, the photoconductivitywas examined. As a result, each of them exhibited a satisfactoryphotoconductivity of 10⁻⁵ to 10⁻⁴ s/cm. Further, each of the resultanta-Si films was examined with respect to deterioration in thephotoconductivity by exposing it to pseudo sun light of AM 1.5 with 100mW in intensity for 130 hours. As a result, the initialphotoconductivity was not substantially reduced.

                                      TABLE 1                                     __________________________________________________________________________    the condition for forming i-type                                              semiconductor layer                                                           photovoltaic introduction of                                                                       substrate                                                                              initial photovoltaic                                                                     deterioration                                                                        hydrogen content              element sample No.                                                                         SiH gas temperature (Ts)                                                                       conversion efficiency                                                                    ratio  of the i-type                 __________________________________________________________________________                                                    layer                         No. 1        intermittent                                                                          higher than Tt                                                                         1.25       0.2     4 atomic %                   (Example 2)                                                                   No. 2        continuous                                                                            higher than Tt                                                                         0.9        0.85   10 atomic %                   (Comparative example 1)                                                       No. 3        intermittent                                                                          higher than Tt                                                                         1.1        0.9    14 atomic %                   (Comparative example 2)                                                       No. 4        continuous                                                                            higher than Tt                                                                         1          1      15 atomic %                   (Comparative example 3)                                                       __________________________________________________________________________

What is claimed is:
 1. A process for forming a silicon-containingamorphous film on a substrate which comprises (a) a step of depositing asilicon-containing amorphous film on said substrate and (b) a step ofirradiating a plasma of inert gas to said silicon-containing amorphousfilm deposited on the substrate in said step (a), wherein said step (a)and said step (b) are alternately repeated while (i) controlling thethickness of said silicon-containing amorphous film deposited on eachrepetition of said step (a) to a thickness of at least 10 Å and (ii)maintaining the temperature of the substrate when irradiating a plasmaof inert gas in step (b) to at least a transition temperature at which amarked reduction is caused in the content of hydrogen atoms contained inthe silicon-containing amorphous film formed on the substrate.
 2. Theprocess according to claim 1, wherein the inert gas is selected from thegroup consisting of He gas, Ne gas, Av gas, Xe gas and Kr gas.
 3. Theprocess according to claim 1, wherein the thickness of thesilicon-containing amorphous film deposited in each repetition of thestep (a) is controlled to be 100 Å or less.
 4. The process according toclaim 1, wherein the substrate is electrically floated upon performingeach repetition of the step (b).
 5. The process according to claim 1,wherein a bias voltage is applied to the substrate upon performing eachrepetition of the step (b).
 6. The process according to claim 5, whereinthe bias voltage is a self-bias voltage provided by the plasma.
 7. Theprocess according to claim 1 wherein the substrate is maintained at atemperature which corresponds to the transition temperature at which amarked reduction is caused in the content of hydrogen atoms contained inthe silicon-containing amorphous film formed on the substrate.
 8. Theprocess according to claim 1, wherein the substrate is maintained at atemperature which is higher than the transition temperature at which amarked reduction is caused in the content of hydrogen atoms contained inthe silicon-containing amorphous film formed on the substrate.
 9. Theprocess according to claim 1, wherein the same inert gas used in eachrepetition of the step (b) is also used in each repetition of the step(a) together with film-forming raw material gas.
 10. The processaccording to claim 9, wherein the concentration of the film-forming rawmaterial gas used in each repetition of the step (a) is controlled to30% or above.
 11. The process according to claim 5, wherein the biasvoltage is a negative bias voltage.
 12. The process according to claim11, wherein the thickness of the silicon-containing amorphous film to beformed in each repetition of the step (a) is controlled to a thicknessin the range of 15 Å to 200 Å.